For those of us who haven't played AD&D, what are those rules?
My recollection from the last time I played AD&D (almost 20 years ago so take with a grain of salt)
you started out as a multi-classed character and could never add a class later on (there was dual classed but it was a different system which let you stop advancing in 1 class to pick up a new one)
non-humans could only multi-class, humans could only dual class (1st ed AD&D allowed a half-elf to dual class solely between the fighter and thief class to allow them to play a bard which used to require levels of fighter and thief to enter it iirc)
You divided your xp evenly between the classes and you received _almost_ the best benefits of each class (hp rolls were divided the number of classes, iirc an elf wizard/fighter could cast spells in armour but a cleric/wizard couldn't and a half-elf wizard/fighter couldn't)
Saving throws were the best of each class for the roll, you used the best attack table or thatc0 progression of your classes etc...
it tended to be problematic partly because of level limits. All non-humans had limited classes they could pick and limited advancement in any class but the thief in the core books (1st ed half-orcs had unlimited advancement in Assassin but I think it had a level limit itself)
generally because of how the level tables worked a multi-classed character was trading 1 level in each class for the abilities of the other class and a wizard/fighter had more hp than a straight fighter but a lot less hp than a straight fighter
In pathfinder it looks like you'd be around 2 levels behind in each class.
I'm not really sure that I'd want to argue that a level 18/18 wizard/fighter
or wizard/cleric isn't clearly ahead of a level 20 wizard
and if you bring level limits in then you've got other problems.
With 3 classes you'd end up with a level 16/16/16 character
a level 16 fighter/cleric/wizard could be a tad overpowered and a level 16 barbarian/ranger/fighter would cause a fair few headaches...
I don't see it as an easy fit for pathfinder